Polyimide resins having heat resistance and improved electrical and mechanical properties have been utilized as insulating protective coatings for electronic parts. Since most polyimide resins are insoluble in organic solvents, polyimide protective coatings are generally prepared by using a solution of polyamic acid which is a polyimide precursor, and applying the solution to substrates, followed by heat curing. This conventional approach, however, has several problems. For example, the resulting polyimide resin coatings show poor adherence to such substrates as nickel, aluminum, silicon, and silicon oxide film.
For improving the adherence to substrates, Japanese Patent Publication (JP-B) Nos. 27439/1968 and 7213/1984 propose polyimide-siloxane copolymers in which a diamine component, which is a reactant for forming polyimide, is partially replaced by a siloxane-containing diamine. Also, JP-B 32162/1983, 32163/1983, and 29510/1989 disclose a method of mixing or reacting a polyamic acid or polyimide precursor with a silane having an amino group or acid anhydride group. However, the former method had the problem that copolymers with an increased content of siloxane showed poor heat resistance. In the latter method, the polyamic acid solution experienced a substantial loss of shelf stability with an increasing amount of silane added.
In order to overcome these problems, JP-B 32162/1983 proposes a composition comprising a polyamide acid having a reactive silicone compound attached at either end and a polydisiloxane having a hydroxyl group at either end. Also, Japanese Patent Application Kokai (JP-A) Nos. 126331/1989 and 126335/1989 propose oligomers obtained by mixing and reacting acid dianhydride, organic diamine, and reactive silicone compound in a suitable proportion. The composition and oligomers are still poor in shelf stability and difficult to dissolve in solvents in high concentrations. The limited concentration solution leaves difficulty in providing sufficiently thick films after curing.